1. Technical Field
The present disclosure relates generally to a microwave antenna that can treat tissue. More particularly, the present disclosure is directed to a microwave antenna suitable for use in resection of tissue.
2. Background of Related Art
Treatment of certain diseases requires destruction of malignant tissue growths (e.g., tumors) or surrounding tissue. It is known that tumor cells denature at elevated temperatures that are slightly lower than temperatures injurious to surrounding healthy cells. Therefore, known treatment methods, such as hyperthermia therapy, heat tumor cells to temperatures above 41° C., while maintaining adjacent healthy cells at lower temperatures to avoid irreversible cell damage. Such methods involve applying electromagnetic radiation to heat, ablate and/or coagulate tissue. Microwave energy is sometimes utilized to perform these methods. In particular, microwave energy is used to coagulate or ablate tissue. Another method used to treat diseased tissue is to resect a portion of the diseased organ, tissue or anatomical structure. For example, a liver may contain diseased tissue and healthy tissue. One treatment option is to pre-coagulate and ablate some of the liver tissue to facilitate resection of a portion of the liver including the diseased tissue. Microwave energy can be used during these types of procedures to pre-coagulate tissue prior to resection, to reduce bleeding during resection and to facilitate the actual resection of the tissue.
The microwave energy may be applied via antennas that can penetrate tissue. There are several types of microwave antennas, such as monopole and dipole antennas. In monopole and dipole antennas, most of the microwave energy radiates perpendicularly away from the axis of the conductor. A monopole antenna includes a single, elongated conductor that transmits the microwave energy. A typical dipole antenna has two elongated conductors parallel to each other and positioned end-to-end relative to one another with an insulator placed therebetween. Each of the conductors is typically about ¼ of the length of the wavelength of the microwave energy making the aggregate length of both conductors about ½ of the wavelength of the microwave energy.
A coaxial dipole antenna typically includes a first elongated conductor and a second elongated conductor disposed concentrically around the first elongated conductor along about half of the distance of the coaxial assembly. The portion having the second elongated conductor is about ¼ of a wavelength and the portion having only the first elongated conductor is also about ¼ of a wavelength, making the aggregate length of the antenna about a ½ wavelength. By selecting the microwave energy wavelength to be twice the length of the dipole, power is more efficiently transmitted from the antenna to the surrounding tissue.
Some microwave antennas have a narrow operational bandwidth, a wavelength range at which operational efficiency is achieved, and hence, are incapable of maintaining a predetermined impedance match between the microwave delivery system (e.g., generator, cable, etc.) and the tissue surrounding the microwave antenna. More specifically, as microwave energy is applied to tissue, the dielectric constant of the tissue immediately surrounding the microwave antenna decreases as the tissue is heated. This drop may cause the optimal microwave energy wavelength to change beyond the bandwidth of the antenna. As a result, there may be a mismatch between the bandwidth of conventional microwave antennas and the microwave energy being applied.